a. Field of the Invention
The present invention relates to catheters and, in particular, to a catheter shaft that facilitates stringing of wires as well as establishing and maintaining proper positioning of wires in a catheter shaft. The invention also provides improved catheter construction techniques.
b. Background Art
Catheters are typically threaded through a blood vessel of a patient to reach a desired site for a medical procedure. For example, in the diagnosis and treatment of atrial fibrillation, a catheter may be routed through a vessel from a patient's leg or neck to access chambers of a patient's heart. Electrodes at the distal end of the catheter can then be used for a variety of purposes including electrical mapping and ablation. The catheter therefore may include one or more internal lumens to accommodate wires (e.g., electrode wires, pull wires for steering or other structures extending through the catheter shaft), as well as to permit irrigation as may be useful for certain procedures.
More specifically, an electrode catheter typically includes a handle set at a proximate end of the catheter, one or more electrodes associated with an electrode tip at the distal end of the catheter and a shaft extending therebetween. The physician uses the hand set to manipulate the catheter and position electrodes at the desired location for the medical procedure. The shaft extends from the handle set to the electrode tip through the patient's blood vessel.
The shaft is typically constructed by extruding layers of polymer onto a core rod. A metal braid may be embedded in the polymer for improved incompressibility. The core is then removed to provide a central lumen. Various wires, for example, electrode and/or pull wires are then threaded through the central lumen. Generally, each wire is threaded through the central lumen and positioned as desired within the lumen. In the latter regard, a specific relative positioning of the pull wires (e.g., diametrically opposed) may be desired for optimal performance. In addition, it may be desired to spatially separate the wires, for example, to reduce the risk of short circuits. Additional liners, cords or other structures (e.g., to define a lumen for irrigation fluids) may be inserted into the central lumen of the catheter shaft, and reflowing of the inner liner of the cathede shaft may be necessary to ensure proper adhesion. It will be appreciated that this processing is complicated and labor intensive. Moreover, there are numerous opportunities for error, which could affect catheter performance.
In addition, the catheter shaft is designed with a number of objectives in mind. First, the shaft is generally dimensioned with an outside diameter that allows the catheter to be threaded through the vessels necessary to perform the desired medical procedures. It is also desired to provide an inside diameter sufficient to accommodate electrode wiring, steering wiring and/or irrigation fluid channels, depending on the intended use of the catheter. Therefore, a limited radial thickness of the catheter shaft is desirable.
At the same time, the shaft should provide certain mechanical properties for optimal functioning. In particular, the shaft should resist compression during use and transmit torque. With regard to resisting compression, it is important for the physician to be able to advance the catheter through the vessel, sometimes against significant frictional resistance, without undue axial compression or snaking of the catheter shaft. Such compression can complicate positioning of the distal end of the catheter at the desired location for a medical procedure. In addition, skilled physicians often rely, to some extent, on tactile feedback to attain and verify proper positioning of the catheter, and such feedback can be impaired by excessive compressibility.
The shaft should also be capable of reliably transmitting torque. In this regard, a physician normally navigates the distal end of the catheter to a desired location in part by turning a handle set at the proximal end of the catheter. Again, substantial frictional forces sometimes resist transmission of torque across the length of the catheter. In some cases, these forces can cause the shaft to twist about a longitudinal axis of the shaft, storing energy in the process in spring-like fashion. If this energy is released suddenly, the distal end of the catheter, which may be bent by a steering mechanism, can be propelled with significant force against unintended tissue. This can have dire consequences in the context of intracardiac procedures.
In order to provide the desired mechanical properties within the noted dimensional constraints, some catheters incorporate a dual braided shaft design involving an inner braided wire and an outer braided wire. Each of braided wires is typically embedded, to some extent, in a polymer so that the braided wire and polymer function as a system to impart desired mechanical properties. More specifically, the inner braided wire system is typically the primary source of compression resistance. The outer braided wire system, having a larger moment arm relative to the longitudinal axis of the shaft, is typically the principal source of torque transmission. Each of the inner and outer braided wire systems may be designed to satisfy its primary function in this regard.
The dual braided shaft is generally formed by extruding a polymer liner onto a rod. The outer braid is then formed on the polymer liner, and an outer polymer jacket is then extruded onto the outer braid. Thereafter, the rod is removed to leave a hollow interior. A coil is then inserted into the hollow interior to form the inner braid, and the polymer liner is reflowed along the length of the shaft to integrate, to some extent, the inner braid into the catheter shaft structure. The various wires and an irrigation tube may then be threaded through the central lumen of the shaft.